The present invention concerns the fabrication of integrated circuits and pertains particularly to the removal of photoresist.
In fabricating integrated circuits, after the use of photoresist from a photolithography or etch step, the photoresist (or resist) must be removed for the subsequent steps. Resist must be removed following a wide variety of processing steps, including dry etching, wet etching, ion implantation, lift-off processes, high temperature postbake and the removal of misaligned resist patterns for reimaging after development and inspection ("rework"). In addition, wafer surface patterns of several different materials may be present under the resist. Typical materials include silicon dioxide (SiO.sub.2), aluminum, polysilicon, silicides and polyimide. It is important when stripping resist to ensure that all the photoresist is removed as quickly as possible without attacking any underlying surface materials.
In the prior art, complete removal of polymers generated during etch of high aspect ratio via holes has proved difficult. The etching processes required to control via sidewall profiles and maintain high selectivity to mask and substrate leave polymers that are very difficult to remove. Residual polymers that are not removed will result in vias having higher resistances. These polymers are typically composed of both organic and inorganic components and may require both dry and wet stripping to achieve complete removal. The more difficult the polymer is to remove, the more aggressive the solvent stripper must be, with consequences in safety, cost, and manageability. The burden of strippability can be lessened by making the postetch ash more effective in creating a soluble polymer, and by increasing the dry etch selectivity to the substrate material. Residual polymers also need to be removed in other applications after photoresist etch.
It has been shown that the OH radicals produced by a plasma reduce the activation energy for resist removal. See Fujimura, S. et al., "Resist Stripping in an O.sub.2 +H.sub.2 O Plasma Downstream," J. Vac. Sci. Technol. B, 9(2), PP. 357-361 (1991). See also U.S. Pat. No. 5,814,155 issued on Sep. 29, 1998 to Mark A. Levan and Ramiro Solis for PLASMA ASHING ENHANCEMENT.